From the answer:
Let me direct you to the support who can provide more details about the QuickBooks Sandbox, qbqa.... You can post your questions about the Sandbox application by following these steps: Go to: https://help.developer.intuit.com/s/ .

From the answer:
Hi there, Kimba, If the last invoice was dated in January 2019, you can start with that record.... Then, inactivate the estimate in 2018 with a memo stating it was inactivated because you started a fresh company file with 2019 data.

From the answer:
Hi there, .... When you inactivate a customer that has an open balance or uncollectable balance in QuickBooks Online (QBO), the system will automatically create an adjusting transaction to offset the balance.